Milkweed butterflies are a subfamily, Danainae, in the family Nymphalidae, or brush-footed butterflies. They lay their eggs on various milkweeds on which their larvae (caterpillars) feed. Historically, this group had been considered a separate family, Danaidae, and the tribes placed herein were sometimes considered distinct subfamilies in the Nymphalidae.
There are some 300 species of Danainae worldwide, but only four are found in North America: the Monarch Butterfly (Danaus plexippus); the Queen (Danaus gilippus); the Tropical Milkweed Butterfly (Lycorea cleobaea); and the Soldier Butterfly (or "Tropic Queen"; Danaus eresimus). Most of the Danaini are found in tropical Asia and Africa, while the Ithomiini are diverse in the Neotropics. Tellervini are restricted to Australia and the Oriental region.
The best known member of this family is the Monarch butterfly. The larvae and the butterflies retain poisonous glycosides from their larval host plant, the milkweed, so they become distasteful to potential predators. These milkweed butterflies (Monarch, Queen, Soldier) eat only milkweeds (Asclepias) as larvae. This highly effective defense strategy shields them against almost all predators that soon learn to avoid these species after attempting to eat them.
Another member known especially for its presence in butterfly greenhouses and live butterfly expositions is the Southeast Asian Idea leuconoe.
Some authors classify this family as a subfamily of Nymphalidae. Many other authors accept the traditional classification of Danaidae as a separate family. This separation is based on similarity of early stages, similar habits and poisonous nature.
Numerous wasps are parasitoids of milkweed butterfly caterpillars.
The extensive modification of landscapes in the United States and Canada, with the removal of roadside weeds that are butterfly host plants and the large-scale use of pesticides, and increased deforestation in Mexico, threatens the migratory Monarch butterfly.
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